Semiconductor device having at least two thin film transistors

ABSTRACT

Two kinds of TFTs are fabricated by the same process with a high production yield to manufacture an active-matrix circuit and a peripheral driver circuit on the same substrate. The active-matrix circuit is required to have a high mobility and a high ON/OFF current ratio. The peripheral driver circuit needs a complex interconnection structure. The active-matrix circuit and the peripheral driver circuit comprising the TFTs are fabricated monolithically. In this step, the gate electrodes of the TFTs of the active-matrix circuit is coated with an anodic oxide on their top and side surfaces. The gate electrodes of the TFTs of the peripheral driver circuit is coated with the anodic oxide on only their top surfaces; substantially no anodic oxide is present on the side surfaces.

This is a divisional application of Ser. No. 08/266,712, filed Jun. 28,1994, now U.S. Pat. No. 5,403,762.

FIELD OF THE INVENTION

The present invention relates to a method of fabricating an insulatedgate semiconductor device or an integrated circuit comprising amultiplicity of insulated-gate semiconductor devices on an insulatingsubstrate with a high production yield and also to a semiconductordevice fabricated by such a method. Semiconductor devices according tothe invention are used as thin-film transistors (TFTs) in a drivercircuit for an active-matrix display such as a liquid crystal display,in a driver circuit for an image sensor, in an SOI integrated circuit,or in a conventional semiconductor integrated circuit (e.g., amicroprocessor, a microcontroller, a microcomputer, a semiconductormemory, or the like). Furthermore, the invention relates to anintegrated semiconductor device having an active-matrix in a broad sense(i.e., which comprises interconnections arranged in rows and columns,and in which a selecting circuit consisting of at least one transistoris disposed in each intersection) and a peripheral circuit for drivingthe active-matrix. In particular, the integrated semiconductor devicecomprises an active-matrix liquid-crystal display (AMLCD), DRAM, SRAM,EPROM, EEPROM, mask ROM, or other semiconductor integrated circuitformed on an insulating substrate.

BACKGROUND OF THE INVENTION

In recent years, researches on formation of insulated-gate semiconductordevices (MOSFETs) on an insulating substrate have been earnestlyconducted. Such formation of a semiconductor integrated circuit on aninsulating substrate is advantageous for high-speed operation, for thefollowing reason. The speed of the prior art semiconductor integratedcircuit has been mainly restricted by the stray capacitance between eachconductive interconnect and the substrate. On the other hand, such straycapacitance does not exist on an insulating substrate. A MOSFET formedon an insulating substrate and having an active layer in the form of athin film is known as a thin-film transistor (TFT). In the prior artsemiconductor integrated circuit, TFTs are used as load transistors foran SRAM, for example.

Recently, a commercial product comprising a semiconductor integratedcircuit which is required to be formed on a transparent substrate hasemerged. Examples of this product include circuits for driving opticaldevices such as liquid crystal displays and image sensors. TFTs are alsoused in these driver circuits. Since these circuits are required to beformed in a large area, there is a demand for a decrease in thetemperature of the TFT fabrication process. Furthermore, where a devicehas numerous terminals on an insulating substrate, if the terminals mustbe connected with a semiconductor integrated circuit, one might considerto form the first stage of the semiconductor integrated circuit or thesemiconductor integrated circuit itself on the same insulating substratemonolithically to increase the packaging density.

Conventionally, the crystallinity of TFTs has been improved by annealingan amorphous, semi-amorphous, or crystallite semiconductor film at atemperature of 450°-1200° C. Thus, a good semiconductor film, i.e.,having sufficiently high mobilities, is obtained. Some amorphous TFTscomprise a semiconductor film made of an amorphous material. However,their mobilities are lower than 5 cm² /V·s, normally as low as about 1cm² /V·s. Their operating speed and inability to fabricate P-channelTFTs have restricted their use severely. Anneal at the above-describedtemperature has been needed to obtain TFTs having mobilities exceeding 5cm² /V·s. Also, the anneal permitted fabrication of P-channel TFTs(PTFTs). These thermal annealing steps can be carried out by the use ofirradiation of laser light or intense light.

However, it has been pointed out that these TFTs do not havesufficiently high reliability to be used in an active-matrix devicebecause of large leakage current in OFF state. Accordingly, we haveproposed improved methods as described in Japanese Patent applicationSer. Nos. 34194/1992 and 30220/1992. Specifically, gate electrodes aremade of a low-resistivity metal such as aluminum. The surface of eachgate electrode is anodized so that the surface is coated with an oxide.Using this lamination of the metal and the oxide as a main mask,impurities are implanted to form an offset region. As a result, theleakage current is reduced. Also, the interlayer insulation is enhancedby the anodic oxide film. Consequently, short circuit at the crossingportions can be greatly reduced.

More specifically, the film of the anodic oxide has only a small numberof pinholes and can withstand high voltages greater than 7 MV/cm. Hence,interlayer insulation is secured. In practice, we have succeeded ingreatly reducing short circuit between interconnections, by utilizingthe techniques described in the above-cited Japanese Patent applicationSer. Nos. 34194/1992 and 30220/1992. This is quite important inactive-matrix regions because interconnections cross each other at verynumerous locations.

However, we have discovered that technically very difficult problemsoccur when one attempts to fabricate a device on which an active-matrixdevice and its peripheral driver circuit are monolithically formed(e.g., a memory or an AMLCD), using the above-described technique.

Generally, a peripheral circuit is complex in structure and hasinterconnects connected in a complex manner. Therefore, even if oneattempts to coat metal electrodes with an anodic oxide, it is impossibleto supply electric power because of the complexity of the interconnectstructure. Also, if interconnects used only for supply of electric powerare formed, then an extra photolithography step is necessary to removethese interconnects. This results in a decrease in the manufacturingyield. If a circuit is constructed together with these extrainterconnects, then the device density is deteriorated severely.

In another proposed method, an anodic oxidation step is used tofabricate an active-matrix circuit portion. The anodic oxidation step isnot employed when a peripheral circuit portion and other regions areformed. This method suffers from a very low production yield. It hasbeen found that the main cause is the presence of numerous pinholesbecause the interlayer insulator is incomplete. The pinholes cause shortcircuits between an upper interconnect and a lower interconnect (i.e., agate electrode and its interconnect).

This is an essential problem where interconnects of a metal having a lowmelting point are used. It is well known that aluminum and their alloysare excellent electrode materials. If impurity atoms are implanted intoan active layer using gate electrodes made of such a material as a maskby a self-aligning process, then activation utilizing thermal annealingat a temperature of 630° C. or higher cannot be adopted. Therefore, itis inevitable that a low-temperature activation technique such as laserannealing is used to activate the impurity atoms. Furthermore, atechnique of forming an interlayer insulator film above 560° C. cannotbe adopted.

For example, an interlayer insulator material such as silicon oxideformed by LPCVD or atmospheric-pressure CVD at a substrate temperatureabove 560° C. contains a quite small number of pinholes. Also, almost noshort circuit occurs between interconnects. However, at low temperaturesbelow 560° C., only sputtering or plasma CVD can be used. In thesemethods, a large amount of dust is deposited onto the film during itsgrowth. This increases the number of pinholes. Also, the resultinginsulation is not satisfactory. Even in a peripheral driver circuit,interconnects cross each other. Therefore, in order to improve theproduction yield, there is a demand for a method of forming an anodicoxide with an improved production yield.

SUMMARY OF THE INVENTION

In view of the foregoing problems with the prior art techniques, thepresent invention has been made.

It is an object of the present invention to provide an optimum devicestructure and an optimum manufacturing process.

The techniques described in the above-cited Japanese Patent applicationSer. Nos. 34194/1992 and 30220/1992 form an anodic oxide. As describedin the patent specifications of these Japanese Patent applications, thegreatest feature of these techniques is that when an inverse voltage isapplied to each gate, the leakage current can be reduced greatly by theoffset effect. This characteristic is necessary for TFTs inactive-matrix regions which operate dynamically because of the necessityof holding pixel voltage with certainty. Also, the characteristic isnecessary where the stand-by electric consumption of a flip-flop circuitshould be suppressed. Therefore, where TFTs of this structure are usedas pixel transistors of an AMLCD, as selecting transistors formingstorage bits of a DRAM fabricated by SOI techniques, or as transistorsforming an inverter circuit for the storage bits of an SRAM (especiallya complete CMOS SRAM), great advantages can be obtained.

However, if peripheral circuits operate statically or partiallystatically, their leakage current presents no serious problems.Therefore, the circuits function satisfactorily even if an anodic oxideis not formed on the side surface, i.e., the offset structure is notadopted.

However, if a dense film as consisting of an anodic oxide is not formedon the top surface of the gate electrode, then leakage betweeninterconnects greatly deteriorates the manufacturing yield. As describedin the above-cited specifications, where laser annealing is utilized,the damage caused by the laser annealing step is minimized by theexistence of an anodic oxide on the top surface of the gate electrode.

We know that a metal aluminum film fabricated by electron-beamevaporation has a flat surface and have submicrometer grain diameters.Therefore, the film well reflects light, especially UV light. After thefilm is directly irradiated with laser light, almost no damage wasobserved. However, after a film fabricated by sputtering or othersimilar method and having large grain sizes of about 1 μm is directlyirradiated with laser radiation, very large damage was observed. Sinceelectron-beam evaporation is not adapted for mass production, thepractical approach is to use sputtering techniques. That is, for aperipheral circuit, the gate electrode needs an anodic oxide not on itsside surface but on its top surface.

Accordingly, in a semiconductor device of the present invention, TFTs inwhich almost no anodic oxide exists on the side surface of each gateelectrode but an anodic oxide is formed only on the top surface areformed together with TFTs of active-matrix regions. TFTs of thisstructure and a device made up of these TFTs are fabricated in themanner described below.

A metal film made of, for example, aluminum is deposited on asemiconductor layer becoming islands and on a gate-insulating film. Anoxide film is formed on the metal film by anodic oxidation. Although thepreferred thickness range of the oxide film depends on the quality ofthe oxide film, we have found that where the thickness of the oxide isless than 30 nm, the composition deviates from the stoichiometric ratio,thus deteriorating the insulation. Therefore, the thickness of theinsulator (the oxide) is preferably 30 nm or more. Since a very highvoltage is applied to the device in order to form the oxide to athickness of 300 nm or more by the anodic oxidation, it is not preferredthat the oxide is formed to a thickness of 300 nm or more.

Then, the oxide and the metal film are etched to form gate electrodes ofa desired shape. Thus, the anodic oxide is left on the top surfaces ofthe gate electrodes, and no anodic oxide exists on the side surfaces inthe peripheral circuit. By the later ion implantation or doping, thegate electrode of the transistor in the peripheral circuit can be madeself-aligned with at least one of source and drain of the transistor inthe peripheral circuit.

The above-described etching process should be a directional etchingprocess such as reaction ion etching (RIE). Where an isotropic etchingprocess is carried out, voids (cusp) are created near the boundarybecause of the difference in etch rate between the anodic oxide and themetal film. As a result, the interconnects extending over the voidseasily break. However, depending on the material, the whole process cannot be effected by RIE.

As an example, where the metal material is aluminum, the anodic oxide isaluminum oxide but this cannot be removed by RIE. Therefore, in thiscase, the aluminum oxide film is first removed by wet etching. Then,using the remaining aluminum oxide as a mask, the metal aluminum isetched by RIE.

If it is impossible to employ RIE for etching of the metal aluminum andthe process relies only on wet etching, then it is desired to make themetal aluminum film as thin as possible. More specifically, the ratiobetween the aluminum oxide film thickness and the metal aluminum filmthickness should be less than 1:3. Preferably, the ratio is less than1:2.

A monolithic matrix circuit is built, using these TFTs, in the mannerdescribed below. A first method comprises the following steps: (1) Ametal film is formed in matrix regions as well as in a peripheralcircuit; (2) This metal film is anodized to form an anodic oxide on thesurface; (3) The anodic oxide is removed from unwanted locations; (4)Using the remaining anodic oxide as a mask, the metal film is etched,and gate electrodes are formed in the peripheral circuit and in thematrix regions; and (5) An electrical current is caused to flow throughthe matrix regions to form an anodic oxide only on the side surface ofeach gate electrode of the matrix regions.

We now take notice of the matrix regions fabricated by this method.First, the anodic oxide is formed by the step (2). The secondanodization is effected by the step (5) while leaving the anodic oxideon the gate electrode. Therefore, a stress is induced between theinitially formed anodic oxide and the anodic oxide formed later. Thismay peel off the initially formed anodic oxide.

This problem can be avoided by adding a step (4') subsequently to thestep (4) as shown in Example 1. This step (4') consists of removing theanodic oxide from the matrix regions while masking only the peripheralcircuit portion. By carrying out this step, the metal material of thegate electrodes of the matrix regions is fully exposed. Then, a uniformanodic oxide is formed on the top and side surfaces by the step (5). Itis easy to mask only the matrix regions. The production yield is notdeteriorated in spite of the addition of the step (4'). However, eventhe gate-insulating film may be etched away, depending on the kind ofthe etchant used. If the semiconductor region surfaces are exposed, thenthe manufacturing yield will be deteriorated. Hence, care must beexercised. In any case, at least two anodization steps are necessary.

A second method according to the invention is the method of Example 2and consists mainly of the following steps: (1) A metal film isdeposited over the whole peripheral circuit portion, and a metal film isformed in the form of gate electrodes on the matrix regions; (2) Anelectrical current is caused to flow through the metal film on theperipheral circuit portion and through the gate electrodes and theirinterconnects of the matrix regions to form an anodic oxide; and (3) Theanodic oxide and the metal film on the peripheral circuit portion areetched, and the gate electrodes of the peripheral circuit portion areformed.

This method involves only one anodization step but at least twophotolithography steps are necessary in order to form the gateelectrodes of the matrix regions and the gate electrodes of theperipheral circuit portion.

The impurities doped are activated in both the peripheral circuit andthe active-matrix circuit by irradiating a laser light thereto.

Instead of this laser irradiation, a thermal annealing at 550° C. orlower may be used to activate the impurities doped. Alternatively, thesubstrate may be heated at a temperature of 200° to 500° C. during thelaser irradiation. Further, these techniques may be combined. Forexample, the thermal annealing may be carried out after or before thelaser irradiation. Further, the thermal annealings may be carried outtwice, i.e. before and after the laser irradiation. The laserirradiation may be carried out maintaining the substrate substantiallyat room temperature. Alternatively, the laser irradiation may be carriedout heating the substrate at a temperature of 200° to 500° C.

It is necessary to control the substrate temperature of the heating inthe laser irradiation or the thermal annealing in order to prevent thegate electrode material from being damaged. The present inventionprevents hillock (abnormal crystal growth) from forming in aperpendicular direction even though the annealing is carried out at arelatively high temperature, since the anodic oxide film is formed onthe top surfaces of the gate electrodes of both the peripheral circuitand the active-matrix circuit (i.e. the pixel transistor circuit).Accordingly, short circuit is hardly formed between layers.

Other objects and features of the invention will appear in the course ofthe description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(E) are cross-sectional views of TFTs according to thepresent invention, illustrating successive steps for fabricating theTFTs; and

FIGS. 2(A) to 2(E) are cross-sectional views of other TFTs according tothe invention, illustrating successive steps for fabricating the TFTs.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

A bottom oxide film 102 having a thickness of 100 to 300 nm was formedon an insulating substrate 101 made of Corning 7059 measuring 300 mm×300mm or 100 mm×100 mm. To form this oxide film, silicon oxide may besputtered within an oxygen ambient. Alternatively, TEOS is decomposed byplasma CVD and a film is deposited. This film is then annealed at 450°to 650° C.

Thereafter, an amorphous silicon film having a thickness of 30-150 nm,preferably 50-100 nm, was deposited by plasma CVD or LPCVD. A protectivelayer consisting of silicon oxide or silicon nitride and having athickness of 20 to 100 nm, preferably 20 to 40 nm, was formed by plasmaCVD. Then, the amorphous silicon film was annealed at 550°-650° C.,preferably at 600° C., for 72 hours to crystallize the amorphous siliconfilm. The crystallinity of the silicon film crystallized in this way wasinvestigated by Raman spectroscopy. A relatively broad peak was observednear 515 cm⁻¹, which differed from the peak (521 cm⁻¹) of a singlecrystal of silicon.

The protective layer was then removed to expose the silicon layer. Thissilicon layer was subsequently photolithographically patterned intoislands to form TFT regions 103 of a peripheral driver circuit and TFTregions 104 of an active-matrix circuit. A gate oxide film (gateinsulating film) 105 having a thickness of 50 to 200 nm was formed onthe islands of the silicon layer by sputtering within an oxygen ambient.Then, an aluminum film 106 having a thickness of 200 nm to 5 μm,preferably 200 to 600 nm, was formed as a conductive film over the wholesurface of the substrate by electron-beam evaporation. This aluminumfilm was oxidized by anodization to form an anodic oxide 107 having athickness of 30 to 300 nm, preferably 100 to 300 nm on the surface. Theconditions under which the aluminum film was anodized were the same asthe conditions used in the techniques described in Japanese Patentapplication Ser. Nos. 30220/1992, 38637/1992, and 54322/1992. Anelectrical current was caused to flow through the aluminum film withinan electrolyte.

Then, as shown in FIG. 1(B), the anodic oxide was etched with an etchantconsisting principally of hydrofluoric acid, e.g., 1/10 buffered HF.Using the remaining anodic oxide as a mask, the metal aluminum film 106was etched by RIE. Then, masking the peripheral circuit regions, theanodic oxide on the gate electrodes of the active-matrix circuit portionwas etched. In this way, the gate electrodes 108 (for NTFTs), 109 (forPTFTs), and gate electrodes 110 of the active-matrix circuit portionwere derived (FIG. 1(B)).

Subsequently, an electrical current was made to flow through only thegate electrodes of the active-matrix circuit portion within anelectrolyte, and an anodic oxide 111 having a thickness of 30 to 300 nm,preferably 200 to 300 nm, e.g., 250 nm, was formed on the top and sidesurfaces of the gate electrodes of the active-matrix circuit under thesame conditions as the foregoing (FIG. 1(C)). By the later ionimplantation, the gate electrode of the transistor in the active-matrixcircuit portion can be offset from at least one of source and drain ofthe transistor in the active-matrix circuit portion.

Then, using the gate electrode portions (i.e., the gate electrodes andthe surrounding anodic oxide film) of the active-matrix circuit and theperipheral driver circuit as a mask, impurity ions were implanted intothe silicon film islands of the TFTs by a self-aligning ion dopingprocess. During this process, phosphorus ions in the form of phosphine(PH₃) were first implanted as a doping gas. Thereafter, only the leftpart of the illustrated island 103 was coated with a photoresist, andboron ions were implanted into the islands 103 and 104, using diborane(B₂ H₆) as a doping gas. The dose of phosphorus ions was 2 to 8×10¹⁵/cm². The dose of boron ions was 4 to 10×10¹⁵ /cm². The dose of boronions was set larger than the dose of phosphorus ions.

Thereafter, using the gate electrodes as a mask, the laminate wasilluminated with a KrF excimer laser light having a wavelength of 248 nmand a pulse width of 20 nsec to recover the crystallinity of theportions which deteriorated due to the ion implantation described above.The energy density of the laser light was 200 to 400 mJ/cm², preferably250 to 300 mJ/cm². As a result, N-type regions 112 and P-type regions113 of a peripheral circuit and P-type regions 114 of an active-matrixcircuit were formed. The sheet resistance of these regions was 200 to800 Ω/square. In this step, instead of irradiation of pulsed laser lightemitted by a KrF excimer-laser, irradiation of intense light producingthe same effects as the irradiation of the pulsed light may be adopted.This method is known as RTA (rapid thermal annealing) used toselectively heat a silicon semiconductor by making use of infraredlight, especially infrared radiation having a peak at wavelengths of 1to 2 μm. Preferably, the infrared light is halogen light of 1.3 μm. Inthis method, the doped regions can be heated to about 1000° to 1200° C.Thus, an anneal can be carried out effectively. Since such infraredradiation is selectively absorbed by the silicon semiconductor, thesilicon film is annealed without thermally affecting the glasssubstrate. Another feature is that irradiation persisting for onlyseveral seconds to several minutes can produce desirable results. Thestructure obtained up to this state is shown in FIG. 1(D).

Then, a silicon oxide film having a thickness of 300 nm was formed as aninterlayer insulator 115 over the whole surface by sputteringtechniques. This insulator may be replaced by a silicon oxide film or asilicon nitride film formed by plasma CVD.

Thereafter, an ITO film was formed on the matrix portion by sputtering.The ITO film was etched to form pixel electrodes 116. Contact holes wereformed on the source/drain electrodes of the TFTs. Chromiuminterconnects 117-121 having a thickness of 800 nm were formed bysputtering and photolithography, as shown in FIG. 1(E). In this case, aninverter circuit is built from NTFTs and PTFTs of the peripheral circuitregions. Finally, the silicon film was annealed at 350° C. withinhydrogen for 2 hours to reduce dangling bonds in the silicon film. Inthis way, the peripheral circuit and the active-matrix circuit werefabricated integrally.

EXAMPLE 2

The process sequence of the present example is illustrated in FIG. 2,(A)-(E). A bottom silicon oxide film 202 having a thickness of 100 to300 nm was formed on an insulating substrate 201 made of Corning 7059measuring 300 mm×300 mm or 100 mm×100 mm. To form this oxide film,silicon oxide may be sputtered within an oxygen ambient. Alternatively,TEOS is decomposed by plasma CVD and a film is deposited. This film isthen annealed at 450° to 650° C.

Thereafter, an amorphous silicon film 203 having a thickness of 30-150nm, preferably 50-100 nm, was deposited by plasma CVD or LPCVD. Aprotective layer 204 consisting of silicon oxide or silicon nitride andhaving a thickness of 20 to 100 nm, preferably 50 to 70 nm, was formedby plasma CVD. Then, as shown in FIG. 2(A), the amorphous silicon filmwas illuminated with light emitted by a KrF excimer laser producing awavelength of 248 nm and a pulse width of 20 nsec to improve thecrystallinity of the silicon film 203. The energy density of the laserlight was 200 to 400 mJ/cm², preferably 250 to 300 mJ/cm². Thecrystallinity of the silicon film 203 crystallized in this way wasinvestigated by Raman spectroscopy. A relatively broad peak was observednear 515 cm⁻¹, which differed from the peak (521 cm⁻¹) of a singlecrystal of silicon. It is advantageous to illuminate the silicon film203 with intense infrared light to promote the crystallization after thecrystallization of the silicon film 203 by the KrF excimer laserirradiation. Since infrared light is selectively absorbed by the siliconsemiconductor, the crystallization of the silicon film 203 can beeffectively accelerated without heating the glass substrate to a largeextent. In particular, defects and dangling bonds can be reduced. Also,crystallization can be carried out, using only irradiation of intenselight. Then, the laminate was annealed at 350° C. for 2 hours in ahydrogen.

Then, the protective layer 204 was removed to expose the silicon layer203. This silicon layer 203 was then photolithographically patternedinto islands to form a peripheral circuit region and an active-matrixcircuit region, in the same way as in Example 1. A gate oxide film(gate-insulating film) was formed on the islands of the silicon layer byeffecting sputtering in an oxygen ambient or by annealing a film at 450°to 650° C., the latter film being obtained by decomposing TEOS by plasmaCVD. Where the latter method is utilized, the high temperature used inthe present step may induce strain in the substrate or cause it toshrink, thus making it difficult to perform later mask alignment.Therefore, where the substrate has a large area, sufficient care must beexercised. Where sputtering techniques are used, the substratetemperature can be set below 150° C. It is desired to anneal the film atabout 300°-450° C. within hydrogen in order to reduce dangling bonds inthe film and to reduce the effects of static charge.

Thereafter, an aluminum film having a thickness of 200 to 500 nm wasformed on the gate-insulating film by sputtering as a conductive film.This film was patterned photolithographically to form a metal aluminumcoating 205 that covers the peripheral circuit region and gateelectrodes 206 of the active-matrix regions and the wirings connectedwith the gate electrodes 206, as shown in FIG. 2(B).

As shown in FIG. 2(C), the substrate was immersed in an electrolyticsolution, and an electric current was caused to flow through thealuminum coating 205 and through the gate electrodes 206 and through thewirings connected with the gate electrodes 206 within an electrolyte,thus forming anodic oxide layers 207 and 208 on their surfaces and ontheir outer peripheries. In the present example, the thickness of theanodic oxide layers was 200 to 250 nm. As a result, the thickness of theremaining metal aluminum was 100 to 400 nm.

Then, as shown in FIG. 2(D), gate electrodes 209, 210 of TFTs of theperipheral circuit region were formed. First, the anodic oxide layer wasetched by wet etching with an etchant consisting principally ofhydrofluoric acid, e.g., 1/10 buffered HF. Then, using the remaininganodic oxide as a mask, the aluminum film was etched with mixed acid.Gate electrodes were formed in this way. As shown, the anodic oxide wasleft on the gate electrodes in the peripheral circuit region. On theother hand, the gate electrodes 211 of the TFTs of the active-matrixregions remained as it was.

Subsequently, using the gate electrode portions (i.e., the gateelectrodes and the surrounding anodic oxide films) of the active-matrixcircuit and the peripheral driver circuit as a mask, impurity ions wereimplanted into the silicon film islands of the TFTs by a self-aligningion doping process. At this time, phosphorus ions in the form ofphosphine (PH₃) were implanted as a doping gas into the whole surface.Then, only the left part of the illustrated peripheral circuit regionwas coated with a photoresist, and boron ions were implanted into theright part of the peripheral circuit region and into the matrix regions,using diborane (B₂ H₆) as a doping gas. The dose of phosphorus ions was2 to 8×10¹⁵ /cm². The dose of boron ions was 4 to 10×10¹⁵ /cm². The doseof boron ions was set larger than the dose of phosphorus ions.

Then, using the gate electrodes of the active-matrix circuit and theperipheral circuit as a mask, the amorphous silicon film was illuminatedwith light emitted by a KrF excimer laser producing a wavelength of 248nm and a pulse width of 20 nsec to crystallize the portions whosecrystallinity was deteriorated by the ion implantation. The energydensity of the laser light was 200 to 400 mJ/cm², preferably 250 to 300mJ/cm². This step may also be carried out by an anneal making use ofirradiation of intense light or infrared light, that is a lightequivalent to a laser light.

As shown in FIG. 2(D), N-type regions 212 and P-type regions 213, 214were formed. The sheet resistance of these regions was 200 to 800Ω/square. Then, a silicon oxide film having a thickness of 300 nm wasformed as an interlayer insulator 215 over the whole surface bysputtering. This silicon oxide film may be replaced by a silicon nitridefilm formed by plasma CVD.

Thereafter, pixel electrodes 216 were fabricated from a transparentconductive material such as ITO on the TFTs of the active-matrixportion. Contact holes were formed on the source/drain of the TFTs.Chromium interconnects 217-221 were formed. In this case, an invertercircuit is built from NTFTs and PTFTs of the peripheral circuit region.Finally, the silicon film was annealed at 350° C. within hydrogen for 2hours to reduce dangling bonds in the silicon film. In this way, theperipheral circuit and the active-matrix circuit were fabricatedintegrally.

[EXAMPLE 3]

This example is shown in FIG. 1. In the same manner as EXAMPLE 1, a TFTregion 103 of a peripheral driving circuit and a TFT region 104 of amatrix circuit are formed of crystalline silicon on a substrate 101 onwhich a base film 102 has been formed. A gate oxide film 105 of 100 nmthickness is formed by a sputtering method in an oxygen atmosphere. Thenan aluminum film 106 of 500 nm thickness is formed on the whole surfaceof the substrate by electron beam deposition method. This is oxidized byan anodic oxidation method, and anodic oxide 107 of 30 to 300 nmthickness, e.g., 100 nm thickness is formed on the surface. (FIG. 1(A))

As is shown in FIG. 1(B), the anodic oxide is etched with an etchantmainly comprising phosphoric acid (e.g., mixed solution of phosphoricacid, nitric acid, and acetic acid added with a minute amount of chromicacid by approximately 3%). The metal aluminum film 106 is etched by RIEwith employing the residual anodic oxide as a mask. Furthermore, a maskis made in the peripheral circuit region, and anodic oxide on the gateelectrode of the matrix circuit is etched. In this manner, gateelectrodes of the peripheral driving circuit 108 (for NTFT) and 109 (forPTFT), and a gate electrode 110 of a matrix circuit portion areobtained. (FIG. 1(B))

After that, electric current is flowed in the gate electrode of thematrix circuit portion only in the same condition as above, anodic oxide111 is formed on the upper surface and the side surface of said gateelectrode by 200 to 300 nm thickness, e.g., 250 nm thickness. (FIG.1(C))

By ion doping method, phosphorus and boron impurities are implanted intothe island silicon film in self-aligning way with employing the gateelectrode portion (i.e., the gate electrode and an anodic oxide filmaround it) as a mask.

While the substrate is heated at 200° to 500° C., e.g., at 350° C., KrFexcimer laser light (wavelength 248 nm, pulse width 20 nsec) isirradiated. Above mentioned impurity region improves crystal characterof the portion of which crystal character has been deteriorated by theion implantation. Energy density of laser is made as 250 to 300 mJ/cm².As a result, an N type region 112 and a P type region 113 of aperipheral circuit, and a P type region 114 of a matrix circuit areformed. Sheet resistance of these regions is 200 to 400 Ω/square.Compared with Embodiment 1, dispersion is less. In Embodiment 1,activation is performed by laser irradiation without particularlyheating the substrate. (FIG. 1(D))

After that, a silicon oxide film of 300 nm thickness is formed on thewhole surface as an interlayer insulator 115 by a plasma CVD method. Apixel electrode 116 is formed of an ITC) film, and chrome wirings 117 to121 (all 800 nm thickness) are formed in source/drain of each TFT. (FIG.1(E))

[EXAMPLE 4]

This example is shown in FIG. 1. In the same manner as EXAMPLE 1, a TFTregion 103 of a peripheral driving circuit and a TFT region 104 of amatrix circuit are formed of crystalline silicon on a substrate 101 onwhich a base film 102 has been formed. A gate oxide film 105 of 100 nmthickness is formed by a sputtering method in an oxygen atmosphere. Thenan aluminum film 106 of 500 nm thickness is formed on the whole surfaceof the substrate by electron beam deposition method. This is oxidized byan anodic oxidation method, and anodic oxide 107 of 30 to 300 nmthickness, e.g. 100 nm thickness is formed on the surface. (FIG. 1(A))

As is shown in FIG. 1(B), the anodic oxide is etched by an etchantmainly comprising phosphoric acid. The metal aluminum film 106 is etchedby RIE with employing the residual anodic oxide as a mask. Furthermore,a mask is made in the peripheral circuit portion, and anodic oxide onthe gate electrode of the matrix circuit is etched. In this manner, gateelectrodes 108 (for NTFT) and 109 (for PTFT) of the peripheral drivingcircuit portion, and a gate electrode 110 of the matrix circuit portionare obtained. (FIG. 1(B))

After that, electric current is flowed in the gate electrode of thematrix circuit portion only. In the same condition as above, anodicoxide 111 is formed on the upper surface and the side surface of saidgate electrode by 200 to 300 nm thickness, e.g. 250 nm thickness. (FIG.1(C))

By ion doping method, phosphorus and boron impurities are implanted intoan island silicon film of each TFT in self-aligning way with employingthe gate electrode portion (i.e., the gate electrode and an anodic oxidefilm around this gate electrode) as a mask.

By performing heat annealing at 300° to 550° C., e.g. at 500° C. for 4hours, doped impurities are activated. Then XeCl excimer laser(wavelength 308 nm, pulse width 20 nsec) is irradiated, thus enhancingactivation. Energy density of laser is made as 250 to 300 mJ/cm². As aresult, an N type region 112 and a P type region 113 are formed in theperipheral circuit, and a P type region 114 is formed in the matrixcircuit. Sheet resistance of these regions is 200 to 400 Ω/square.Dispersion is not observed a lot. Because the source/drain region hasbeen activated to some degree by heat annealing before laserirradiation, laser light is easy to be transmitted. Activation effect byirradiation of laser light gets in a portion deep in the source/drain.(FIG. 1(D))

After that, a silicon oxide film of 300 nm thickness is formed on thewhole surface as an interlayer insulator 115 by plasma CVD method. Apixel electrode 116 is formed of an ITO film, and chrome wirings 117 to121 (all 800 nm thickness) are formed in source/drain of each TFT. (FIG.1(E))

In this embodiment, not only laser light irradiation but also heatannealing is utilized to activate doped impurities. Thus a device withhigh reliability with very little trap level in NI junction or PIjunction between source/drain and channel is obtained.

[EXAMPLE 5]

This example is shown in FIG. 2. In the same manner as EXAMPLE 2, anamorphous silicon film 203 (50 nm thickness) and a silicon oxide orsilicon nitride film 204 (50 nm thickness) are formed on a substrate 201on which a base oxide film 202 has been formed. KrF excimer laser light(wavelength 248 nm, pulse width 20 nsec) is irradiated, and a siliconfilm 203 is crystallized. (FIG. 2(A))

The protection film 204 is removed, and a silicon film 203 is exposed.This silicon film is patterned into islands, thus a peripheral circuitregion and an active matrix region are formed. Nextly, a gate oxide filmis formed. An aluminum film of 500 nm thickness is formed by a sputtermethod, and is patterned. As is shown in FIG. 2(B), a metal aluminumfilm 205 covering the peripheral circuit region and a gate electrode 206of the active matrix region are formed. (FIG. 2(B))

As is shown in FIG. 2(C), electric current is flowed to said aluminumfilm 205 and the gate electrode 206 with the substrate being immersed inan electrolyte solution. Anodic oxide films 207 and 208 are formed onthe surface or around the aluminum film 205 and the gate electrode 206.In this embodiment, the thickness of the anodic oxide film is 200 to 250nm. As a result, the thickness of the remaining metal aluminum is 200 to400 nm. (FIG. 2(C))

As is shown in FIG. 2(D), gate electrodes 209 and 210 of TFT of theperipheral circuit are formed. First of all, the anodic oxide film isetched with an etchant mainly comprising phosphoric acid (e.g., mixedsolution of phosphoric acid, nitric acid, and acetic acid added with aminute amount of chromic acid). Thereafter, the metal aluminum is etchedwith phosphoric acid with the remaining anodic oxide as a mask. In thismanner, a gate electrode is formed. However, as is shown in the figure,anodic oxide is resided on these electrodes of the peripheral circuit.On the other hand, the gate electrode 211 of TFT of the active matrixregion remains as it is.

By ion doping method, phosphorus and boron impurities are implanted intoan island silicon film of each TFT in self-aligning way with employingthe gate electrode portion (i.e., the gate electrode and an anodic oxidefilm around it) as a mask.

While the substrate is heated at 200° to 500° C., e.g., at 400° C., KrFexcimer laser light (wavelength 248 nm, pulse width 20 nsec) isirradiated. Above mentioned impurity region improves crystal characterof the portion of which crystal character has been deteriorated by theion doping. By performing heat annealing at 300° to 550° C. thereafter,crystal character is further improved. (FIG. 2(D))

As a result, as is shown in FIG. 2(D), an N type region 212 and P typeregions 213 and 214 are formed. Sheet resistance of these regions is 200to 400 Ω/square. Dispersion is not observed a lot. In the heat annealingabove mentioned, junction between source/drain and channel formationregion to which laser light is difficult to penetrate, that is, Pijunction and NI junction are also activated. Thus characteristic isimproved.

After that, a silicon oxide film of 300 nm thickness is formed on thewhole surface as an interlayer insulator 215 by a plasma CVD method. Apixel electrode 216 is formed of an ITC) film, and chrome wirings 217 to221 (all 800 nm thickness) are formed in source/drain of each TFT. (FIG.2(E))

In this embodiment, heat annealing is utilized to activate dopedimpurity. Thus a device with high reliability with very little traplevel in PI junction or NI junction is obtained.

[EXAMPLE 6]

This example is shown in FIG. 2. In the same manner as EXAMPLE 5, anamorphous silicon film 203 (50 nm thickness) and a silicon oxide orsilicon nitride film 204 (50 nm thickness) are formed on a substrate 201on which a base oxide film 202 has been formed. KrF excimer laser light(wavelength 248 nm, pulse width 20 nsec) is irradiated, and a siliconfilm 203 is crystallized. (FIG. 2(A))

The protection film 204 is removed, and a silicon film 203 is exposed.This silicon film is patterned into islands, thus a peripheral circuitregion and an active matrix region are formed. Nextly, a gate oxide filmis formed. An aluminum film of 500 nm thickness is formed. As is shownin FIG. 2(B), a metal aluminum film 205 covering the peripheral circuitregion and a gate electrode 206 of an active matrix region are formed.(FIG. 2(B))

As is shown in FIG. 2(C), anodic oxide films 207 and 208 are formed inthe same manner as EXAMPLE 5 by anodic oxidation. The thickness of theanodic oxide film is 200 to 250 nm. (FIG. 2(C))

The anodic oxide film 207 and the aluminum film 205 are etched, and thengate electrodes 209 and 210 of TFT of the peripheral circuit region areformed as is shown in FIG. 2(D).

By ion doping method, phosphorus and boron impurities are implanted intoan island silicon film of each TFT in self-aligning way with employingthe gate electrode portion (i.e., the gate electrode and an anodic oxidefilm around it) as a mask.

To activate the impurities, heat annealing is performed at 300° to 550°C., e.g., at 500° C. for four hours. KrF excimer laser light (wavelength248 nm, pulse width 20 nsec) is irradiated to activate the dopedimpurities further, while keeping the substrate temperature at 200° to500° C., e.g., 400° C. Characteristic of crystal is further improved byperforming heat annealing at 300° to 550° C. e.g., 500° C. for fourhours again. By the heat annealing after laser irradiation, defectsgenerated in the device by the laser irradiation can be decreased. As aresult, as is shown in FIG. 2(D), an N type region 212 and P typeregions 213 and 214 are formed. (FIG. 2(D))

A silicon oxide film of 300 nm thickness is formed on the whole surfaceas an interlayer insulator 215 by a plasma CVD method. A pixel electrode216 is formed of an ITC) film, and chrome wirings 217 to 221 (all 800 nmthickness) are formed in source/drain of each TFT. (FIG. 2(E))

The present invention makes it possible to fabricate TFTs having largeOFF resistivities in active-matrix regions. Also, peripheral circuitregions having a complex structure can be fabricated with a highproduction yield. Furthermore, both kinds of regions can be manufacturedmonolithically by the same process. Accordingly, with respect to anAMLCD, the cost could be saved by more than 30% compared with theconventional case in which TAB connections were made. Conventional TFTshaving high mobilities have the disadvantage that sufficient OFFresistivities cannot be obtained. Therefore, where a high-temperatureprocess is used and TFTs of the same structure are used for both matrixand peripheral circuits, for example, the function of any one kind ofTFTs would have been sacrificed.

On the other hand, in the method described in the above-cited JapanesePatent application Ser. No. 34194/1992 or 30220/1992, ideal TFTs havinghigh mobilities and high OFF resistivities can be obtained.Consequently, an image can be displayed without difficulty even wherehigh-speed operation and a high ON/OFF current ratio are required asencountered in a digital gray scale for which we filed patentapplications (e.g., Japanese Patent application Ser. Nos. 169306/1991and 209869/1991).

However, it is very difficult to fabricate a complex peripheral drivercircuit from TFTs described in the above-cited Japanese Patentapplication Ser. No. 34194/1992 or 30220/1992. The present invention hascompletely solved these difficulties. Especially, the circuitconfiguration for the digital gray scale representation described aboveis quite complex because it requires more complex signal processing thanordinary display methods. Therefore, in the past, it has been requiredthat the peripheral driver circuit be connected with ICs with TABconnections only because it is difficult perform an anodization step.

The present invention makes it possible to fabricate any complexperipheral circuit in such a way that its capability is hardlydeteriorated. Of course, the invention is not limited to digital grayscale display. The invention can also be applied to LCDs utilizingordinary analog gray scale representation. Especially, the best use ofthe invention is made where it is applied to an LCD with highinformation content displaying more than 1000 rows. In the illustratedexamples, AMLCDs have been described. Obviously, the invention can beapplied with similar utility to any device comprising an active-matrixcircuit and a peripheral driver circuit in a broad sense (e.g., DRAMsand SRAMs). In this way, the invention is industrially quiteadvantageous.

What is claimed is:
 1. A semiconductor device comprising:(a) a firsttransistor comprising: a first semiconductor region; a first gateelectrode provided adjacent to said first semiconductor region with afirst gate insulating film therebetween; and a first oxide of a materialof said first gate electrode provided on side and top surfaces of saidfirst gate electrode, and (b) a second transistor comprising: a secondsemiconductor region; a second gate electrode provided adjacent to saidsecond semiconductor region with a second gate insulating filmtherebetween; and a second oxide of a material of said second gateelectrode provided only on a top surface of said second gate electrode,wherein substantially no oxide of said material of said second gateelectrode is provided on a side surface of said second gate electrode.2. The device of claim 1 wherein said material of said first gateelectrode and said material of said second gate electrode comprise aconductive material.
 3. The device of claim 2 wherein said conductivematerial comprises a metal.
 4. The device of claim 2 wherein saidconductive material comprises aluminum.
 5. The device of claim 1 furthercomprising an insulating material which is different from said firstoxide and said second oxide and is provided in contact with said sidesurface of said second gate electrode.
 6. The device of claim 1 whereinsaid device is an active-matrix display, a microprocessor, amicrocontroller, a microcomputer or a semiconductor memory.
 7. Asemiconductor device comprising:(a) a first group of transistors eachcomprising: a first semiconductor region; a first gate electrodeprovided adjacent to said first semiconductor region with a first gateinsulating film therebetween; and a first oxide of a material of saidfirst gate electrode provided on side and top surfaces of said firstgate electrode, and (b) a second group of transistors each comprising: asecond semiconductor region; a second gate electrode provided adjacentto said second semiconductor region with a second gate insulating filmtherebetween, and a second oxide of a material of said second gateelectrode provided on only a top surface of said second gate electrode;wherein substantially no oxide of said material of said second gateelectrode is provided on a side surface of said second gate electrode.8. The device of claim 7 wherein said material of said first gateelectrode and said material of said second gate electrode comprise aconductive material.
 9. The device of claim 8 wherein said conductivematerial comprises a metal.
 10. The device of claim 8 wherein saidconductive material comprises aluminum.
 11. The device of claim 7further comprising an insulating material which is different from saidfirst oxide and is provided in contact with said side surface of saidsecond gate electrode.
 12. The device of claim 7 wherein said device isan active-matrix display, a microprocessor, a microcontroller, amicrocomputer or a semiconductor memory.
 13. A semiconductor devicecomprising:an insulating substrate; a plurality of transistors providedon the insulating substrate and having gate electrodes and interconnectscomprising a conductive material; an active-matrix portion composed of afirst group of the transistors, gate electrodes of the transistors ofthe first group each having a film comprising an oxide of saidconductive material on side and top surfaces thereof; and a peripheraldriver circuit portion composed of a second group of the transistors, agate electrode of each of the transistors of the second group having afilm comprising said oxide of said conductive material only on a topsurface of said gate electrode and having substantially no oxide of saidconductive material on a side surface of said gate electrode.
 14. Thedevice of claim 13 wherein said conductive material comprises a metal.15. The device of claim 13 wherein said conductive material comprisesaluminum.
 16. The device of claim 13 further comprising an insulatingmaterial which is different from said oxide of said conductive materialand is provided in contact with said side surfaces of said gateelectrodes of the transistors of the second group.
 17. The device ofclaim 13 wherein said device is an active-matrix display, amicroprocessor, a microcontroller, a microcomputer or a semiconductormemory.